Nanoscale synthesis has traditionally relied on generating nanomaterials from bulk precursors using a number of excellent though imperfect approaches. For instance, various “top-down” strategies, such as milling, imprinting, or etching techniques, are limited with respect to the available geometries, shapes, and sizes of synthesizable nanomaterials that can be efficiently generated. (Mirkin et al., MRS Bull. 2001, 26, 506; Xia et al., Adv. Mater. 2003, 15, 353.) In addition, diverse “bottom-up” methodologies starting from either atomic or molecular precursors in the gaseous or solution phase often are unable to yield simultaneous control over nanoparticle structure, surface chemistry, monodispersity, crystal structure, and assembly. (Dloczik et al., Nano Lett. 2003, 3, 651; Patzke et al., Angew. Chem., Int. Ed. 2002, 41, 2446.)
It would be conceptually easier to control the chemical structure of matter at the nanometer scale if one were able to start with, transform, and subsequently manipulate nanoscale precursors to obtain the desired target materials. One exciting strategic approach aimed at fulfilling this objective is associated with the use of localized solid-state chemical transformations via the insertion (Cao et al., J. Solid State Chem. 2004, 177, 2205; Gates et al., J. Am. Chem. Soc. 2001, 123, 11500), exchange (Dloczik et al., Nano Lett. 2003, 3, 651; Son et al., Science 2004, 306, 1009), or deletions (Armstrong et al., Angew. Chem., Int. Ed. 2004, 43, 2286; Zhu et al., J. Am. Chem. Soc. 2005, 127, 6730) of individual atoms. In other words, existing nanostructures serve as structural templates from which nanomaterials of a diverse nature and a complex composition, which may be difficult or otherwise impossible to synthesize, can be readily generated. Collectively, these types of reactions with minor alterations could be used to produce many technologically important, nanometer-scale crystalline materials, with a wide range of size- and shape-tunable properties. (Dloczik et al., Nano Lett. 2003, 3, 651; Cao et al., J. Solid State Chem. 2004, 177, 2205; Gates et al., J. Am. Chem. Soc. 2001, 123, 11500; Son et al., Science 2004, 306, 1009; Armstrong et al., Angew. Chem., Int. Ed. 2004, 43, 2286; Zhu et al., J. Am. Chem. Soc. 2005, 127, 6730; Burda et al., Chem. Rev. 2005, 105, 1025.) The main point involved is that classes of new nanomaterials can be created through reasonably straightforward in situ localized structural transformations, which are often modifications of versatile bulk reactions.
As a model system to demonstrate this idea, nanocrystallites of TiO2 (titania) are of great interest for photocatalysts, gas sensors, pigments, and photovoltaic applications, because of their electronic, optoelectronic, and catalytic properties, which are intrinsically coupled to their high surface area, porosity, low cost, and chemical stability. (Hoffmann et al., Chem. Rev. 1995, 95, 69.) Hence, it is not surprising that groups have been highly motivated to synthesize titania nanostructures by solution chemistry methods, involving either titanium sulfates, titanium tetrahalides, titanium alkoxides, or other organometallic titanium derivatives, under various experimental conditions, such as the presence of either acidic and alkaline media. (Hoffmann et al., Chem. Rev. 1995, 95, 69; Li et al., J. Am. Chem. Soc. 2005, 127, 8659; Zhang et al., Nano Lett. 2001, 1, 81.)
However, there is limited precedence for producing titania nanostructures from an existing nanoscale motif. For instance, TiO2(B) nanowires have been prepared by heating acid-washed titanate nanowires at 400° C. for 4 h in air; the titanates in that case were initially generated by adding anatase TiO2 to a highly concentrated aqueous NaOH solution. (Armstrong et al., Angew. Chem., Int. Ed. 2004, 43, 2286.) These TiO2(B) nanowires could be further transformed into their anatase one-dimensional (1-D) analogues as well as into rod-shaped rutile grains between 600 to 800° C. and at 900° C., respectively. (Yoshida et al., Solid State Chem. 2005, 178, 2179.) Platelike BaTiO3 and anatase particles can be synthesized from an H+-form of titanate (e.g., H1.07Ti1.73O4.nH2O) with a lepidocrocite-like layered structure using a hydrothermal soft chemical synthetic process. (Feng et al., Chem. Mater. 2001, 13, 290.) Titanate nanostructures can be converted into their anatase and rutile TiO2 nanoparticle polymorphs in simple wet-chemical conditions in acidic aqueous dispersions. (Zhu et al., J. Am. Chem. Soc. 2005, 127, 6730.)
A recently reported study from the Alivisatos group aimed to rationally dictate the size and shape of the resultant nanoscale product in selenide nanocrystal systems. (Son et al., Science 2004, 306, 1009; Burda et al., Chem. Rev. 2005, 105, 1025.) However, before the present invention, rationally controlling the size and shape of resultant titania nanoscale products have not yet been reported.
Another relevant area of focus in nanotechnology involves the preparation of higher-order assemblies, arrays, and superlattices of various, individual nanostructures. The preparation of organized assemblies of inorganic materials has tended to rely on the use of organic ligands, additives, or templates. (Whitesides et al. Science 2002, 295, 2418; Bartl et al., Acc. Chem. Res. 2005, 38, 263; Park et al., Science 2004, 303, 348; Caruso, F. Adv. Mater. 2001, 13, 11; Colfen et al., Angew. Chem., Int. Ed. 2003, 42, 2350; Sanchez et al., Nat. Mater. 2005, 4, 277.)
However, before the present invention, preparation of higher-order assemblies of titanate and titania nanostructures have not yet been provided, particularly, such preparation has not been achieved without the use of templates.